Fluid pressure drive units have been used in many fields because a driving force of fluid pressure can be obtained easily if only a power source is present. For example, the fluid pressure drive unit can be used for angle adjustment around the vertical axis of a snowplow blade of a snow removal device that is detachably mounted in front of a vehicle.
One example is the snow removal device described in Patent Document 1. FIG. 3 of the current application is a hydraulic circuit diagram of a snow removal device that is background art to the present invention.
A snow removal device 70 includes a hydraulic pump 51 driven by an electric motor M, a pair of single-acting cylinders 52A and 52B that make angle adjustments (right and left angle adjustments) around the vertical axis of a snow plow blade 61, a pressure-oil tank 53, a change valve 54 for changing over the supply of hydraulic oil sent from the hydraulic pump 51 to the single-acting cylinder 52A or 52B, an overload relief valve 55 used in the case of overload, and a main relief valve 56 that determines the specified working pressure of the snow removal device 70.
The snow removal device 70 further includes a change valve 62 for changing over the supply of hydraulic oil sent from the hydraulic pump 51 to the right and left angle adjustment side or the up and down adjustment side of the snow plow blade 61, a single-acting cylinder 63 for up and down adjustment, a return valve 64 for returning the hydraulic oil to the pressure-oil tank 53 when the single-acting cylinder 63 lowers, and a controller 65 for angle adjustment and up and down adjustment.
With the above-described configuration, the snow removal device 70 can make not only right and left angle adjustments but also up and down position adjustments to the snow plow blade 61 by utilizing the hydraulic drive.
For the snow removal device 70, the two single-acting cylinders 52A and 52B are used to make the right and left angle adjustments to the snowplow blade 61. However, to reduce the cost and to make the unit compact, one single-rod double acting cylinder should be used if possible. For this purpose, it is necessary to solve a problem in that the quantity of hydraulic oil differs between the bottom side and the rod side of the cylinder.
As a solution to the problem that the quantity of hydraulic oil differs between the bottom side and the rod side of the cylinder where one single-rod double acting cylinder is used, the hydraulic drive unit described in Patent Document 2 (proposed by the applicant of the present invention) can be used. The basic configuration and basic operation of this hydraulic drive unit are explained below.
FIG. 4 is a hydraulic circuit diagram of a fluid pressure drive unit that is background art to the present invention.
A hydraulic drive unit OU includes a hydraulic pump OP that sends hydraulic oil under pressure in both of the forward and reverse directions by means of a forward/reverse rotation motor M to impart a hydraulic driving force to a driven body W independently as hydraulic oil is circulated in a closed system. The system's basic components also include a hydraulic actuator OA (herein, a hydraulic cylinder) that is operated by the hydraulic oil to generate the driving force, a tank OT for storing the hydraulic oil in a closed space, an operate check valve OC that controls the flow of hydraulic oil in both of the forward and reverse directions between the hydraulic pump OP and the hydraulic actuator OA, and a change valve OI that controls the flow of hydraulic oil in both of the forward and reverse directions between the hydraulic pump OP and the tank OT.
The operate check valve OC includes a pair of check valves OCa that basically allow only the flow of hydraulic oil from the hydraulic pump OP to the hydraulic actuator OA, and a pair of pilot lines OCb that pilot the hydraulic oil pressure sent to one check valve OCa to the other check valve Oca.
These paired check valves OCa are provided in a pipe line that connects one port of the hydraulic pump OP and a bottom-side fluid chamber OAa of the hydraulic actuator OA to each other and in a pipe line that connects the other port of the hydraulic pump OP and a rod-side fluid chamber OAb of the hydraulic actuator OA to each other.
The change valve OI connects and disconnects either of the pipe lines between the hydraulic pump OP and the bottom-side fluid chamber OAa of the hydraulic actuator OA and between the hydraulic pump OP and the rod-side fluid chamber OAb of the hydraulic actuator OA to and from the tank OT.
In the explanation below, a left-hand side element in the figure of, for example, the pair of right and left check valves OCa, is sometimes referred to as a “bottom side” element because it relates to the hydraulic oil going into and out of the bottom-side fluid chamber OAa of the hydraulic actuator OA. An element on the right-hand side of the figure is thus sometimes referred to as a “rod side” element because it relates to the hydraulic oil going into and out of the rod-side fluid chamber OAb. Similarly, regarding the port of the hydraulic pump OP, the left-hand side is sometimes referred to as the bottom side, and the right-hand side as the rod side.
In a hydraulic drive unit OU configured as described above, when the hydraulic pump OP is stopped, the outflow of hydraulic oil from both of the bottom-side fluid chamber OAa and the rod-side fluid chamber OAb of the hydraulic actuator OA is inhibited, and the position of the hydraulic actuator OA is maintained against a given external force.
When the hydraulic pump OP is rotated so that hydraulic oil is discharged to the bottom-side port, the hydraulic oil is supplied from the hydraulic pump OA to the bottom-side fluid chamber OAa after passing through the bottom-side check valve OCa. At the same time, the rod-side check valve OCa is pushed open by the hydraulic oil pressure of the bottom-side pilot line OCb to allow the hydraulic oil to flow out of the rod-side fluid chamber OAb and into the hydraulic pump OP. This produces a flow of hydraulic oil circulating clockwise between the hydraulic pump OP and the hydraulic actuator OA, so that a driving force in the extension direction is generated in the hydraulic actuator OA.
As this is occurring, when the hydraulic actuator OA is a hydraulic cylinder as shown in the figure the amount of hydraulic oil flowing out of the rod-side fluid chamber OAb is less than the amount of hydraulic oil flowing into the bottom-side fluid chamber OAa by an amount corresponding to the volume of the rod of the piston with according to the displacement of the piston in the hydraulic cylinder. However, pressure from the bottom-side hydraulic oil changes the change valve OI over so that the pipeline to the rod-side fluid chamber OAb is connected to the tank OT, and hydraulic oil is thereby supplied from the tank OT to make up for this deficiency.
On the other hand, when the hydraulic pump OP is rotated so that hydraulic oil is discharged to the rod-side port, a circulation flow of hydraulic oil reverse to the above is produced, and therefore a driving force in the contraction direction is generated in the hydraulic actuator OA. An excess of hydraulic oil flows from the bottom-side fluid chamber OAa into the hydraulic pump OP. However, this excess hydraulic oil is returned to the tank OT because the pipe line to the bottom-side fluid chamber OAa is connected to the tank OT by the action of the change valve OI reverse to the above, and oil is thereby returned to the tank OT.
Depending on the position of the piston in the hydraulic cylinder (the hydraulic actuator OA), the quantity of hydraulic oil in the closed tank OT increases or decreases, and the pressure of the gas enclosed in the tank OT fluctuates. By making the volume of the enclosed gas proper, the fluctuations in gas pressure are prevented from influencing the operation of the hydraulic drive unit OU.
Thus, even though the hydraulic actuator is used in a closed system and there is a difference between the quantities of hydraulic oil going in and out of the cylinder, the function of the hydraulic drive unit OU is accomplished and maintained.
The hydraulic drive unit OU is provided with additional elements described below in addition to the basic elements described above.
Slow return valves SR that throttle only the flow of hydraulic oil from the fluid chambers OAa and OAb to the check valves OCa are provided in the pipe lines between the bottom-side fluid chamber OAa and the rod-side fluid chamber OAb of the hydraulic actuator OA and the check valves OCa of the operate check valve OC.
The slow return valve SR prevents hunting occurring when an external force is applied by the driven body W during the operation of the hydraulic pump OP.
A pipeline provided with a relief valve RV1 branches from the pipeline between the slow return valve SR and the check valve OCa to the tank OT. A similar pipeline provided with a relief valve RV2 branches to the tank OT from the pipeline between the hydraulic pump OP and the check valve OCa on the bottom side and the rod side.
The relief valves RV1 and RV2 let excessive hydraulic oil escape to the tank OT if an abnormal pressure occurs in the unbranched pipeline.
Another pipeline, this one provided with an emergency manual valve MV, branches to the tank OT from the pipeline between the bottom-side slow return valve SR and the check valve OCa. If the hydraulic pump OP stops because a power source is not present, the emergency manual valve MV opens the pipelines of the bottom-side fluid chamber OAa and the rod-side fluid chamber OAb of the hydraulic actuator OA to the tank OT so that the hydraulic actuator OA can be operated manually.
Using the configuration described above, even in the case where an abnormality occurs while the basic function of the hydraulic drive unit OU is attained properly, the hydraulic drive unit OU secures safety, reliability, and accident avoiding properties by preventing the abnormality from leading to damage to the unit OU.
When one attempts to use the hydraulic drive unit OU as it is for right and left angle adjustment of the snow plow blade of the snow removal device described above, and an excessive external force W acts suddenly due to large amounts of snow or obstacles while right or left angle adjustments are made to the snow plow blade and the angle is held (at this time, the hydraulic pump OP is stopped), the relief valve RV1 can return the hydraulic oil in one oil chamber with an excessively high pressure to the tank T, but cannot at the same time take in the deficient hydraulic oil in the other oil chamber with a negative pressure from the tank T. Therefore, a negative pressure is produced in the hydraulic actuator (cylinder) OA, and an unstable state in which the piston position cannot be held sometimes occurs, so that improvement of these situations has been desired.
That is to say, there are times when the angle of the snow plow blade is changed by an excessive external force W while the unit OU is not being operated. It is desirable then that the snow plow blade not be unstable, i.e., that the angle of the blade be unchanged after the excessive external force is removed. However, this need has not been met by the conventional hydraulic drive unit OU.
There has been a need for a hydraulic drive unit in which a stable position is held without operating the hydraulic pump OP even after an external force higher than the rated specification has acted on the hydraulic drive unit OU during a time when the load or the angle (position) is held, that is when the hydraulic pump OP is stopped.
Also, this problem is not limited to a unit using oil pressure, and is widely common to fluid pressure equipment.    [Patent Document 1] U.S. Pat. No. 3,706,144 (FIG. 1, reference numerals 76, 82, 88, FIG. 2)    [Patent Document 2] Japanese Unexamined Patent Application Publication No. 2006-132604 (FIG. 11, Paragraphs [0004] [0021])